Method of growing single crystal of β-barium borate

ABSTRACT

In growing a single crystal of β-BaB 2  O 4  from a melt of BaB 2  O 4  by the Czochralski method, crushed single crystal particles of either β-BaB 2  O 4  or α-BaB 2  O 4  are used as the starting material of the melt. The primary advantage of using the crushed single crystal particles resides in that transformation of a polycrystal initially nucleated on a platinum rod, which is brought into contact with the melt in place of a seed crystal, to single crystal can be accomplished in a greatly shortened time. In consequnce, high quality single crystals are obtained at good yield.

BACKGROUND OF THE INVENTION

This invention relates to a method of growing a single crystal of β-barium borate, β-BaB₂ O₄, by a Czochralski technique.

It is known to grow a high purity β-BaB₂ O₄ single crystal by the Czochralski method from a BaB₂ O₄ melt. The starting material for the melt is either a powder of β phase BaB₂ O₄ (Journal of Crystal Growth, Vol. 106 (1990), 728-731) or sintered grains of β phase BaB₂ O₄ powder (Journal of Crystal Growth, Vol. 114(1991), 676-682).

However, practical applications of the known growth methods encounter several problems. When the starting material of the melt is a β phase powder, which may be sintered, it is convenient to use a platinum rod instead of a seed crystal. At the contact of the platinum rod with the melt a crystalline material nucleates on the platinum rod, but the nucleated material is a polycrystal of β-BaB₂ O₄. To cause transformation of the nucleated polycrystal to a single crystal there is the need of first adjusting the diameter of the polycrystal to about 1.5 mm and then growing the crystal to a length of 5 to 10 mm while keeping the diameter of about 1.5 mm. This operation takes a considerably long time and requires intricate techniques, and often the operation fails by breaking of the growing crystal from the melt or by accidental transition of the β phase to the α phase. Besides, the yield of acceptable β-BaB₂ O₄ single crystals is relatively low because in many cases grain boundaries exist in the grown β-BaB₂ O₄ single crystals.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved method for efficiently growing a high quality single crystal of β-BaB₂ O₄ from a melt of BaB₂ O₄ by a Czochralski technique.

According to the invention the object is accomplished by using crushed single crystal particles of either β-BaB₂ O₄ or α-BaB₂ O₄ as the starting material of the melt.

When crushed single crystal particles of β-BaB₂ O₄ are used as the starting material, the melt partly retains the BaB₂ O₄ structure even after the lapse of a considerable time from the moment of melting. Therefore, a β-BaB₂ O₄ polycrystal which is very good in crystal orientation nucleates on a platinum rod brought into contact with the melt, and by virtue of the good orientation the transformation of the polycrystal to single crystal can be accomplished in a fairly short time. Furthermore, the probability of existence of subgrain boundaries in the fully grown crystal becomes very low, and hence high quality β-BaB₂ O₄ single crystals can be otained at good yield.

It may seem queer to use β-BaB₂ O₄ single crystal particles for growing another single crystal of β-BaB₂ O₄. However, in industrial practice it is inevitable that some portions of grown single crystals have defects such as cracks or subgrain boundaries or become useless. In the present invention it is possible to utilize such defective or useless single crystals of β-BaB₂ O₄ by crushing into suitable particles.

When crushed single crystal particles of α-BaB₂ O₄ are used, the melt also partly retains the BaB₂ O₄ structure for a considerably long time. Therefore, the transformation of the aforementioned polycrystal to single crystal can be accomplished in a short time, and high quality β phase single crystals can be obtained at good yield. It is very convenient to use α phase single crystals since α-BaB₂ O₄ single cyrstals of fairly large size can easily be grown at good yield by a conventional Czochralski method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

As the starting material for the growth of a single crystal of β-BaB₂ O₄ by the Czochralski method, crushed particles of defective or useless single crystals of β-BaB₂ O₄ were used. The crushed single crystals were originally grown by the Czochralski method using a β-BaB₂ O₄ powder as the starting material.

Using a conventional RF furnace, the crushed particles of the β phase single crystals were heated in a platinum crucible. Initially the temperature was raised at a rate of about 200° C./min until the temperature neared the melting point of β-BaB₂ O₄. Then the heating rate was slowed down to 10° C./min, and heating was continued in order to completely melt the crystal particles with care to avoid overheating. At the end of this melting operation the temperature in the center of the melt was about 1100° C.

Immediately after the above melting operation, a vertically held platinum rod having a diameter of 1.5 mm was brought into contact with the melt to initiate crystallization of a polycrystal on the tip of the platinum rod. Then the temperature of the melt was carefully lowered by about 10°-20° C. in order to enlarge the diameter of the polycrystal on the platinum rod. After that the platinum rod was pulled up at a rate of 3 mm/hr. When the pull-up reached about 7 mm the polycrystal turned into a single crystal having a diameter of 3 mm. In this operation 3-4 hr elapsed from the beginning of precipitation of the polycrystal on the platinum rod until the change of the polycrystal to the single crystal. After that the pulling operation was continued while the temperature of the melt was lowered in order to increase the diameter of the single crystal.

The product of the above crystal growth process was a crack-free β-BaB₂ O₄ single crystal 12 mm in diameter and 50 mm in length. This single crystal was cut and polished to provide optical surfaces for surveyllance with a striameter. No striae were detected, and the single crystal proved to be an optically high quality crystal.

Example 2

As the starting material, crushed single crystal particles of α-BaB₂ O₄ were used. The single crystals were originally grown by the Czochralski method using a BaB₂ O₄ powder as the starting material.

Melting of the α-BaB₂ O₄ single crystal particles in a platinum crucible and growth of a crystal from the melt were carried out in the same manner as in Example 1. In this example too a platinum rod having a diameter of 1.5 mm was used as a cold finger for seeding.

The product of this example was a crack-free β-BaB₂ O₄ single crystal 10 mm in diameter and 30 mm in length. This single crystal proved to be an optically high quality crystal free of striae. In this example, it took 3-4 hr to accomplish transformation of the initially nucleated polycrystal to single crystal.

Comparative Examples

As the starting materials for the growth of β-BaB₂ O₄ single crystals by the Czochralski method, (i) a BaB₂ O₄ powder, (ii) a powder mixture of B₂ O₃ and BaO in the proportions equivalent to the composition of BaB₂ O₄ and (iii) a powder mixture of H₃ BO₃ and BaCO₃ in the proportions equivalent to the composition of BaB₂ O₄ were alternately used. In every case the crystal growth process was generally similar to that in Example 1.

In any of these comparative examples the polycrystal initially precipitated on the platinum wire was inferior in crystal orientation, and it took more than 10 hr to change the polycrystal to single crystal. In every case the obtained β-BaB₂ O₄ single crystal had cracks and subgrain boundaries. 

What is claimed is:
 1. A method of growing a single crystal of β-barium borate from a melt of BaB₂ O₄ by a Czochralski technique, characterized in that crushed single crystal particles of α-barium borate are used as the starting material of the melt.
 2. A method according to claim 1, comprising the steps of bringing a platinum rod into contact with said melt to thereby precipitate a polycrystal of β-barium borate on the platinum rod, transforming the polycrystal to a single crystal and thereafter growing the single crystal. 